I have a peace lily that was left by a formal employee, I adopted the poor thing, which was almost dead, and have tried to take care of it. It has started new growth and continues to put out new leaves, but when they reach a height of about 8-10 inches they start to have these brown areas around the edges and then wither.

My guess, and it is only a guess like all these diagnoses done 'blind', is that the plant has been severely over-fertilized. Pass a lot of water through the soil to wash out the excess salts from the fertilizer and then go back to normal thorough watering and see if that helps. house plants need only occasional very dilute feedings. Derek

I have a peace lily I only water(soak) once or maybe twice a week It seems to be doing pretty well. Do the brown marks look kind of like burn marks,, if so it may be getting to much sunlight. Mine likes indirect sunlight. Meaning it doesn't like sunlight directly on it leaves. I just found that out from my neighbor who works at a nursery. Good luck I'm still trying to figure out some of my plants too.